In recent years, water-absorbent resins having a high water absorbency are developed and frequently used mainly for disposable uses, for example, as absorbent articles (e.g. disposable diapers and sanitary napkins) and further as water-retaining agents for agriculture and horticulture and for industrial sealing materials. As to such water-absorbent resins, many monomers and hydrophilic polymers are proposed as their raw materials. Of them, acrylic water-absorbent resins as obtained from acrylic acid and/or its salt as the monomers are industrially most commonly used because of their high water absorbency.
Because water-absorbent resins are generally used for disposable uses (such as disposable diapers), it is essential that they are inexpensive. Therefore, the enhancement of their productivity is in high demand. In addition, there is a natural high demand for the absorbent articles to avoid problems with respect to the safety and coloration of the absorbent articles. Specifically, the water-absorbent resin contains the unreacted residue of acrylic acid. Although the content of the unreacted acrylic acid is several hundred to about 1,000 ppm by weight, a decrease in the content of the unreacted acrylic acid is demanded. In addition, the water-absorbent resin is combined with white pulp in the absorbent articles. Therefore there is a high demand for the water-absorbent resin also to be white so as not to give any foreign-substance feeling or appearance caused by coloration.
In addition, the water-absorbent resin is water-swellable and water-insoluble. However, as described in Patent document 1, in the water-absorbent resin, there is also contained an uncrosslinked water-soluble polymer (water-soluble component) in the range of several wt % to several tens of wt %. The decrease of the content of this water-soluble component is also demanded. Moreover, as described in Patent document 2, the absorbent articles containing the water-absorbent resin is required to possess acceptable water absorption properties under pressure, such as absorption capacity under pressure and liquid permeability under pressure.
In order to solve the above problem, there has been suggested the process including the step of polymerizing a monomer having impurities in small amounts to produce a water-absorbent resin. Examples of such a process include: the process including the step of carrying out purification so that a monomer has a heavy metal content of not more than 0.1 ppm and the step of carrying out polymerization of the monomer (Patent document 3); the process including the step of carrying out polymerization by using acrylic acid including acrylic dimmer or oligomer in small amounts (Patent documents 4 and 5); the process including the step of carrying out purification of acrylic acid for polymerization to obtain an acetic acid or proprionic acid content of less than 400 ppm (Patent document 6); the process including the step of carrying out polymerization by using acrylic acid including protoanemonin in small amounts (Patent document 7); the process including the step of carrying out polymerization by using acrylic acid including furfural in small amounts (Patent document 8); and the process including the step of carrying out polymerization by using acrylic acid including hydroquinone in small amounts (Patent document 9). As the process including the step of reducing the amount of impurities in material for a water-absorbent resin, the following processes have been suggested. That is, the process including the step of treating acrylic acid with an aldehyde treatment agent (Patent document 10), and the process including the step of treating acrylate with an active carbon (Patent document 11).
As disclosed in Patent documents 3 through 11, there has been suggested the method for realizing a water-absorbent resin with excellent properties by the process including the step of purifying acrylic acid or the like as raw material at a high purity. However, there occurs the cost problem and the problem of decrease in productivity.
Furthermore, there has been suggested the polymerization process for a water-absorbent resin, including the step of adding trace components in certain amounts for the improvement in properties of the resulting water-absorbent resin. Examples thereof includes: the process in which acrylic acid has a methoxyphenol content of 10 to 200 ppm (Patent document 12); the process in which there coexists furfural of 11 to 2000 ppm (Patent document 13); and the process using metal (Patent documents 14 and 15). However, in Patent documents 12 and 13, there occurs the problem that the resultant water-absorbent resin becomes colored (turns yellow) due to oxidation of methoxyphenol and furfural, which are contained in a monomer, in the course of the production of a water-absorbent resin.
[Patent document 1]
U.S. Pat. No. 4,654,039
[Patent document 2]
U.S. Pat. No. 5,562,646
[Patent document 3]
Japanese Unexamined Patent Publication No. 31306/1991 (Tokukaihei 3-31306)
[Patent document 4]
Japanese Unexamined Patent Publication No. 211934/1994 (Tokukaihei 6-211934)
[Patent document 5]
International Publication WO04/52949
[Patent document 6]
International Publication WO03/95510
[Patent document 7]
European Patent No. 1302485
[Patent document 8]
U.S. Patent Application Publication No. 2004/0110913
[Patent document 9]
U.S. Pat. No. 6,444,744
[Patent document 10]
International Publication WO03/14172
[Patent document 11]
International Publication WO04/52819
[Patent document 12]
U.S. Patent Application Publication No. 2004/0110914
[Patent document 13]
U.S. Patent Application Publication No. 2004/0110897
[Patent document 14]
U.S. Pat. No. 5,439,993
[Patent document 15]
European Patent No. 1457541